Text thread disappearing and unable to send texts.

After sending a picture to my son's phone, as an attachment in a text which he received, the entire thread of our texts over four years (since I bought this phone) has disappeared from my phone and now I am unable to send him a text. I can receive texts from him, but when I try to text him I receive an 'Unable to send text' notification. I can text other phones as always. Can you help please? My phone is a HTC Desire S.
Many thanks